Although we don’t know yet if hearing loss causes dementia, if it does, it is something that we can treat in late life—with the potential to have a great impact on delaying dementia. —Jennifer Deal, PhD
Explore This IssueSeptember 2019
How Hearing Loss Might Cause Cognitive Decline
Several theories exist as to how hearing loss might be a causative factor of cognitive decline. One possible mechanism relates to the additional cognitive load placed on the brain when it has to work harder to process impoverished auditory input from the ear, said Marlan R. Hansen, MD, professor in the departments of otolaryngology–head and neck surgery and neurosurgery at the University of Iowa, in Iowa City. In this theory, the brain of someone with hearing loss reallocates resources that could otherwise be used for memory, thinking, or other functions to simply try to make sense of distorted sound and speech signals. Recent studies have shown that hearing loss places additional demands on the brain, including recruitment of areas of the brain not normally activated by sound processing in normal hearing patients (Ear Hear. 2018;39:204–214; Ear Hear. 2016;37 Suppl 1:5S–27S; J Neurosci. 2003;23:3423–3431).
A second possible mechanism by which hearing loss may cause dementia and cognitive decline is through changes in brain structure and function, Dr. Deal added. Neuroimaging studies suggest that hearing loss may affect the brain, even in regions outside the primary auditory cortex (J Neurosci. 2011;31:12638–12643; Front Hum Neurosci. 2018;12:172). Individuals with hearing loss appear to recruit executive networks and show evidence of cross-modal plasticity between the somatosensory and auditory systems for compensatory processing of degraded acoustic signals. Hearing loss has also been associated with lower gray matter volume in the primary auditory cortex and with faster rates of brain atrophy over time in the temporal lobe and whole brain.
A third possible link between hearing loss and cognitive decline is the observation that significant levels of hearing loss lead to decreased social interactions, Dr. Hansen said. This can further exacerbate structural or functional changes in the brain due to hearing loss. Social isolation negatively impacts a variety of health conditions, including cardiovascular health, that are likewise linked to cognitive decline and dementia.
Dr. Breen said that he frequently hears reports from patients and their family members that progressive hearing loss leads to patients isolating themselves, avoiding social situations, and forgoing activities they previously found enjoyable and stimulating. Correlations between depression and cognitive decline have been reported as well.